Balanced, low impedance, high frequency transmission line

ABSTRACT

A plurality of insulated equal diameter conductors are clustered in a ring about a centered idler conductor. By providing a center idler conductor insulated from the outer conductors, while connecting only every other one of the outer conductors in parallel between first and second terminals and connecting only the remaining outer conductors in parallel between third and fourth terminals, a transmission line of low impedance, balance and symmetry is provided.

United States Patent 1191 McClure et al. I

1 1 BALANCED, LOW IMPEDANCE, HIGH FREQUENCY TRANSMISSION LINE [75] Inventors: Donald Avery McClure,

Pennsauken; Harvey Clay Nichols, Barnsboro, both of NJ.

[73] Assignee: RCA Corporation, New York, N.Y. I [22] Filed: July 27, 1973 121 1 Appl. No.-: 383,110

. 174/115, 333/97 R [51] Int. Cl. H0lp 5/00v [58] Field of Search 333/1, 4, 5 24 R, 84 R, 333/97 R; 174/32, 33, 34, 36, 113 R, 113 C, l 15 [56] References Cited UNITED STATES PATENTS 1,625,125 4/1927 Latour 174/32 UX 51 June 4, 1974 Primary Examiner-Paul L. Gensler Attorney, Agent, or Firm-Edward J. Norton; Robert 4 L. Troike I ABSTRACT By providing a center idler conductor insulated from the outer conductors, while connecting only every other one of the outer conductors in parallel between first and second terminals and connecting only the remaining outer conductors in parallel between third and fourth terminals, a transmission line of low impedance, balance and symmetry is provided.

9 Claims, 8 Drawing Figures BALANCED, LOW IMPEDANCE, HIGH FREQUENCY TRANSMISSION LINE The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Navy.

BACKGROUND OF THE INVENTION This invention relates to transmission lines and particularly to balanced, high frequency transmission lines of low characteristic impedance.

There are many applications'in which it is necessary to transfer relatively high frequency power (for example, signals in the MHz frequency region and above) from a balanced device such as a (push-pull) output power amplifier to a following stage or device several inches or feet away. Such applications can require that the transmission line used to complete the transfer be of a low characteristic impedance, for example, 25 ohms or less.

A commonly used form of balanced transmission line is the two paralleled conductor transmission line, identified in connection with television lead-in cable as ftwin lead." This form of balanced transmission line BRIEF DESCRIPTION OF THE INVENTION A balanced, low impedance transmission line is provided for completing a connection between a first and a second pair of terminaIs'The transmission line includes a center conductive wire or, alternatively, a centralized slender, elongated body (vermiform body) having a conductive surface. A plurality of even numbered, insulated, substantially equal diameter wires are peripherally clustered in a ring about this center wire or body. Only every other one of the equal diameter wires are connected together at first and second tenninal points at opposite ends of the equal diameter wires to connect these wires in parallel. The remaining peripherally clustered equal diameter wires are connected together at third and fourth terminal points to connect only these wires in parallel. The center wire or body is insulated from the clustered wires to act as an idler conductorat the center of the transmission line.

DETAILED DESCRIPTION A more detailed description follows in conjunction with thefollowing drawing wherein:

FIG. 1 is a cross-sectional view of a transmission line according to one embodiment of the present invention;

F IG. 2 is a perspective view of a portion of the transmission line of FIG. 1;

FIG. 3 is a sketch illustrating the transmission line of FIG. 1 connected between utilizing devices;

FIG. 4 is a sketch illustrating the capacitances in the transmission line according to FIG. 1;

FIG. 5 is a cross-sectional view of the transmission line of FIG. 1 including shielding;

.FIG. 6 is a cross-sectional view of a transmission line according to a second embodiment of the present invention;

FIG. 7 is a cross-sectional view of a transmission line according to a third embodiment of the present invention; and

FIG. 8 is a perspective view of the center vermiform body in FIG. 7.

Referring to FIGS. I and 2, a transmission line 10 comprises seven insulated conductive wires ll, l3, I5, 17, 19,21 and 23. Conductive wire 11 has an insulating sheathing lla covering the continuous length of the conductor 11b. Similarly, wire 13 has an insulating sheathing 13a over conductor 13b; wire 15 has an insulating sheathing 15a over conductor 15b; wire 17 has an insulating sheathing 17a over conductor 17b; wire 19 has an insulating sheathing 19a over conductor 19b; wire 21 has an insulating sheathing 21a over conductor 21b; and wire 23 has, an insulating sheathing 23a over conductor 23b. By way of example, the'conductive wires 11 through 23 may be No. 22HF. The wires 11 through 23 with the insulating sheathing each have a maximum overall diameter of about 0.0281 inches. The diameter of each conductor 11b through 23b without insulation is about 0.025 inches. The insulation thickness in each case is about 0.003] inches. The conductors 1 lb through 23b are soft drawn copper, and the insulating sheathing, referred to as FORMVAR, is a vinylacetal varnish. This type of wire is supplied by Phelps-Dodge Magnet Wire Company. Ft. Wayne, Ind. or by Essex International, Inc., Magnet Wire Division, Ft. Wayne, Ind.

The diameters of the conductive wires 1 I through 23, with or without the insulating sheathing, are equal. The conductive wire 11 is placed at the center of the transmission line 10 with the other six wires 13 through 23 clustered about wire 11 as shown in FIG. I. The insulat-' ing sheathing covering each one of the six peripherally arranged wires 13 through 23 is in contact with the insulating sheathing of the two adjacent peripherally arranged wires and with'the insulating sheathing of the central wire 11.

Referring to FIG. 3, there is illustrated the manner in which this balanced, low impedance line is coupled between utilizing devices 27 and 29. Device 27 has a pair of coupling terminals 27a and 27b and device 29 has a pair of coupling terminals 290 and 29b. The idling wire 11 in FIG. 2 remains unconnected to either of the terminals 27a or 27b of device 27 or terminals 29a or 29b of device 29. The terminals 27a and 29a can be at one potential (positive, for example) when the other terminals 27b and 29b are at a different potential (negative in the example given). The conductors of every other one of the wires 13'through 23 are connected together in parallel. The conductive wires l3, l7 and 21 are connected to each other at about their opposite ends at terminals 27a and 290. This may be done by twisting the ends of the wires l3, l7 and 21 together at the ends. The conductors of the remaining three'wires l5, l9 and 23 are connected together at about their opposite ends at terminals 27b and 29b in a similar fashion. The wires l3, l7 and 21 are therefore in parallel and assume at one cross-sectional point along the line one instantawhere L equals the inductance per unit line length and C equals the capacitance per unit line length. The introduction of an unconnected or idler wire at the center of the line increases the capacitance between the other wires. A drastic increase in the interwire capacitance is achieved by the structure shown in FIGS. 1 and 2.'

FIG. 4 represents an exploded view of the seven wire line of FIGS. 1 through 3, illustrating the capacitance relationships. In FIG. 4, the l circle represents the idler wire 11 in FIG. 1, and the circles marked and represent the potential' of the conductors of wires .13 through 23 in FIG. I at one instant of time. There is, in addition to the capacitances C, through C between the conductors of the peripheral wires 13 through 23, additional capacitance C 7 through. C between the conductor of the central idler wire 11 and the conductor of each of the peripheral wires 13 through 23.

Measurements comparing a simple twisted pair line with a seven wire line made of the wire stock described above show that there is a 5.91 increase in ca pacitance per unit length for the seven wire structure. The twisted pair characteristic impedance was 39 ohms and that of the seven wire line was approximately 7 ohms, giving a squared impedance ratio of 31.04. The inductance per unit length must have decreased by a factor of 5.25 in the seven wire case.

A further lowering of the characteristic impedance can be attained by enclosing the line within a closely spaced conductive outer shield 31 as shown in FIG. Without the shielding, however, measurements indicate that there remains little radiation to, or pick-up from, nearby wires or circuitry, provided the line is terminated in its eharaete fistie iinpedancefThis indicates an excellent balance to ground. Enhancement of the balance and improvement in mechanical stability may be achieved by moderately twisting the transmission line along its longitudinal axis.

In a second embodiment shown in FIG. 6, the insulated wires 13 through 23 are peripherally mounted about a central idler wire 11 which is completely of conductive material, the diameter of the non-insulated, central wire II in FIG. 6 being equal to the diameter of the wires 13 through 23 including the insulating sheathing.

In another embodiment of the invention as illustrated inFlG. 7, the insulated, continuous conductor, central wire 11 of FIG. 1 is replaced by a vermiform body with a center portion 41 of insulating dielectric material. The outer surface 43 of the body is plated with conductive material either continuously in the longitudinal di-,

rection or in separated segments. By breaking the conductive outer surface of the body 11 at segments 45,

47, 49 by gaps in the plating, as shown in FIG. 8, the.

creation of a current path along the surface of the center body 11 the length of the transmission line is prevented. The diameter of the vermiform body 11 including the dielectric center portion 41 and the plated conductor or conductors 43 equals the diameter of the peripheral wires 13 through 23 including the insulating sheathing. I

What is claimed is:

1. A balanced, low impedance transmission line for propagating signals at relatively high radio frequencies comprising, in combination:

a vermiform body extending along the centerof said transmission line having at least a substantial conductive surface,

' a plurality of even numbered insulated substantially equal diameter wires peripherally clustered in a ring about said body with said conductive surface insulated from the conductive portion of said wires and with said wires extending along the length of said transmission line, i

- means at first and second terminal points located at substantially opposite ends of said equal diameter wires to connect only every other one of said equal diameter wires in parallel, and

means at third and fourth terminal points located at substantially opposite ends of said equal diameter wires to connect only the remaining of said equal diameter wires in parallel,

said body being insulated from said wires along the length of said transmission line.

2. The combination claimed in claim I wherein the thickness and dielectric constant of the insulative sheathing of said insulated equal diameter wires is selected to provide enhanced capacitance between said equal diameter wires and between the-conductive surface of said body and said equal diameter wires at said high radio frequencies.

3. The combination claimed in claim 2 wherein said vermiform body is a continuous conductor and an insulative sheathing surrounds the conductive surface of said body.

continuous conductor and insulated sheathing thereon form a structure the diameter of which equals the diameter of said insulated equal diameter wires.

5. The combination claimed in claim I wherein said plurality of equal diameter wires is six.

6. The combination claimed in claim 1 wherein said vermiform body is a wire conductor of all conductive material having a diameter equal to that of said equal diameter wires including the insulation.

7.- The combination claimed in claim 1 wherein said vermiform body has a dielectric center portion substantially surrounded by a conductive portion and the diameter of the dielectric portion and the conductive portion equals the diameter of the equal diameter wires including the insulation.

8.-The combination claimed in claim 1 wherein said equal diameter wires are given a moderate twist along the lengthwise axis.

9. The combination claimed in claim 1 including a conductive shield about the insulated equal diameter wires coaxial with said vermiform body.

4. The combination claimed in claim 3 wherein said 

1. A balanced, low impedance transmission line for propagating signals at relatively high radio frequencies comprising, in combination: a vermiform body extending along the center of said transmission line having at least a substantial conductive surface, a plurality of even numbered insulated substantially equal diameter wires peripherally clustered in a ring about said body with said conductive surface insulated from the conductive portIon of said wires and with said wires extending along the length of said transmission line, means at first and second terminal points located at substantially opposite ends of said equal diameter wires to connect only every other one of said equal diameter wires in parallel, and means at third and fourth terminal points located at substantially opposite ends of said equal diameter wires to connect only the remaining of said equal diameter wires in parallel, said body being insulated from said wires along the length of said transmission line.
 2. The combination claimed in claim 1 wherein the thickness and dielectric constant of the insulative sheathing of said insulated equal diameter wires is selected to provide enhanced capacitance between said equal diameter wires and between the conductive surface of said body and said equal diameter wires at said high radio frequencies.
 3. The combination claimed in claim 2 wherein said vermiform body is a continuous conductor and an insulative sheathing surrounds the conductive surface of said body.
 4. The combination claimed in claim 3 wherein said continuous conductor and insulated sheathing thereon form a structure the diameter of which equals the diameter of said insulated equal diameter wires.
 5. The combination claimed in claim 1 wherein said plurality of equal diameter wires is six.
 6. The combination claimed in claim 1 wherein said vermiform body is a wire conductor of all conductive material having a diameter equal to that of said equal diameter wires including the insulation.
 7. The combination claimed in claim 1 wherein said vermiform body has a dielectric center portion substantially surrounded by a conductive portion and the diameter of the dielectric portion and the conductive portion equals the diameter of the equal diameter wires including the insulation.
 8. The combination claimed in claim 1 wherein said equal diameter wires are given a moderate twist along the lengthwise axis.
 9. The combination claimed in claim 1 including a conductive shield about the insulated equal diameter wires coaxial with said vermiform body. 